Pulse width modulation can be implemented in any one of four variations. In each case, an input signal is modulated in such a manner that the width of a pulse within a stream of pulses represents an amplitude of the input signal at a sampling time. In a first variation of PWM, the relative time of the rising edge of each pulse in the stream of pulses is fixed, and each pulse is varied in width by changing the timing of its falling edge. In a second variation of PWM, the relative time of the falling edge of each pulse in the stream of pulses is fixed, and each pulse is varied in width by changing the timing of its rising edge. In a third variation of PWM, the relative time of the center of each pulse in the stream of pulses is fixed, and each pulse is varied in width by changing the timing of both its rising and falling edge in a symmetrical manner. Finally, in a fourth variation of PWM, the relative time of the center of each pulse in the stream of pulses is fixed, and each pulse is varied in width by independently changing the timing of both its rising and falling edge. Each of these variations of PWM can be implemented either as class AD (a two state implementation) or as class BD (a three state implementation).
Generally speaking, most higher frequency PWM systems have been implemented using analog circuitry due to the high level of computational complexity in accurately processing the edges of the pulses in the pulse stream at high speeds. This is particularly true when both edges are varied as in the third and fourth variation, and is most difficult when both edges are varied independently. Moreover, the higher the frequency of processing required, the greater the current consumption that digital circuits use frequently dictating that analog PWM circuits be used, particularly in battery powered communication equipment.
Analog pulse width modulators generally create a stream of pulses by use of a comparator that compares an input signal with a periodic sawtooth or triangular waveform, thus producing a stream of pulses whose accuracy depends upon the accuracy and reproducibility of the sawtooth or triangular waveform. It would be desirable to have a Pulse Width Modulator with certain improved capabilities.